Method of coating high atomic number metals onto oxygen or hydroxyl rich surfaces

ABSTRACT

Methods for making layered materials and layered materials including high atomic number metals and metal alloys adhered to surfaces are provided. Such surfaces may be oxygen or hydroxyl rich surfaces. Certain methods include depositing a tie down layer of a first metal or metal alloy particles onto a first surface of base material and depositing a high atomic number metal or metal alloy layer onto the first surface after depositing the tie down layer, wherein particles comprising the high atomic number metal or metal alloy layer have a higher atomic number than the first metal or metal alloy particles.

CROSS-REFERENCE TO RELATED PATENT APPLICATION(S)

This patent application claims the benefit of and priority to62/878,057, filed on Jul. 24, 2019, the contents of which are herebyincorporated by reference in their entirety for any and all non-limitedpurposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention described herein was made in the performance of work undera NASA contract and by an employee of the United States Government andis subject to the provisions of the National Aeronautics and Space Act,Public Law 111-314, § 3 (124 Stat. 3330, 51 U.S.C. Chapter 201), and maybe manufactured and used by or for the Government for governmentalpurposes without the payment of any royalties thereon or therefore.

BACKGROUND OF THE INVENTION

Layering of dissimilar materials on one another through deposition ordiffusion bonding requires a strong interface between the dissimilarmaterials for a resulting layered material to have good adhesion andmechanical strength at the interface. High atomic number materials, suchas metals and alloys, have a tendency to make brittle oxides at oxygenrich metal and carbon surfaces. For example, high atomic number metalsoften form brittle surface oxides when coated on oxygen and hydroxylrich surfaces. As such, high atomic number materials, such as metals andalloys, have presented problems when used in forming layered materials,especially layered materials where dissimilar materials interface withone another. One example of such poor adhesion, is the poor adhesiontypically experienced with the layering of a refractory metal onto glassfiber. As a specific example, when tantalum (Ta) is radio frequencyplasma sprayed onto glass fabric using current methods, the Ta easilyflakes off of the glass fabric during cutting, handling, and folding ofthe glass fabric.

While some high atomic number metals have been shown to bond strongly tocarbon fiber fabric under certain conditions, bonding of high atomicnumber metals or metal alloys to oxygen rich surfaces, such as glassfabrics, has presented significant challenges. For example, tantalum(Ta) has been shown to bond strongly to carbon fiber fabric and thisstrong adhesion can be attributed to the tantalum carbon bonding to formcarbides. In comparison, tantalum has been shown to bond poorly withoxygen rich surfaces, such as glass fabric. The tantalum easily flakesoff of the glass fabric during cutting, handling, and folding of theglass fabric. Without being bound of any specific of potentiallynumerous problems facing the art, the poor adhesion of tantalum withoxygen rich surfaces, such as glass fabric, could be attributed to theformation of brittle oxides having inferior adhesion.

Accordingly, embodiments disclosed herein address one or moredeficiencies in the art.

BRIEF SUMMARY

Aspects disclosed herein provide methods to improve the adhesion of highatomic number metals and alloys to various surfaces, such as forexample, oxygen or hydroxyl rich surfaces, inclusive of but not limitedto: such as oxygen or hydroxyl rich metal surfaces, oxygen or hydroxylrich films, and/or oxygen or hydroxyl rich fabrics.

Various embodiments may be wholly or partially be implemented in themaking of a layered material. Various embodiments may include depositinga tie down layer of a first metal or metal alloy particles onto a firstsurface of base material and depositing a high atomic number metal ormetal alloy layer onto the first surface after depositing the tie downlayer.

Further aspects relate to layered materials. Novel layered materials mayinclude a base material having an oxygen or hydroxyl rich first surfaceand a high atomic number metal or metal alloy layer bonded to the firstsurface at an interface, wherein the interface includes particles of ametal or metal alloy of a lower atomic number metal.

Various embodiments may include metal tie down layers of metal or metalalloy particles, such as metal tie down layers formed from titanium,palladium, tin, bismuth-tin alloy, antimony-tin alloy, molybdenum,molybdenum alloy, palladium alloy, aluminum, aluminum alloy, antimony,antimony alloy, titanium alloy, iron, iron alloy, and/or copper-tinalloy. Various embodiments may include high atomic number metal or metalalloy layers deposited after the metal tie down layers, such as highatomic number metal or metal alloy layers formed from tantalum, tantalumalloy, tungsten, tungsten alloy, lead, lead alloy, bismuth, and/orbismuth alloy.

These and other features, advantages, and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims, andappended drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitutepart of this specification, illustrate exemplary embodiments of theinvention, and together with the general description given above and thedetailed description given below, serve to explain the features of theinvention.

FIG. 1 is a photograph showing a comparison of a sample radio frequency(RF) plasma sprayed tantalum (Ta) coated glass fiber fabric without atie down layer to an embodiment sample RF plasma sprayed Ta coated glassfiber fabric with a RF plasma sprayed tie down layer of Titanium (Ti).

FIG. 2 is a process flow diagram that may be partially or whollyimplemented in the formation of a layered material having strongadhesion between a high atomic number metal and/or metal alloy layer andan oxygen and/or hydroxyl rich layer using a lower atomic number metaltie down layer in accordance with one embodiment.

FIG. 3 is a photograph of two samples of RF plasma sprayed Ta coatedglass fiber fabric with a RF plasma sprayed tie down layer of Ti inaccordance with an embodiment.

FIG. 4 is a scanning electron microscope (SEM) image of a Ta/Ti glassfabric surface in accordance with an embodiment.

FIG. 5 is a SEM image of a cross-section of the Ta/Ti glass fabric ofFIG. 4 .

FIG. 6 is another SEM image of a cross-section of the Ta/Ti glass fabricof FIG. 4 .

FIG. 7 is a SEM image of a cross-section of an individual glass fiber ofthe Ta/Ti glass fabric of FIG. 4 .

DETAILED DESCRIPTION

For purposes of description herein, it is to be understood that thespecific devices and processes illustrated in the attached drawings, anddescribed in the following specification, are simply exemplaryembodiments of the inventive concepts defined in the appended claims.Hence, specific dimensions and other physical characteristics relatingto the embodiments disclosed herein are not to be considered aslimiting, unless the claims expressly state otherwise.

The various embodiments will be described in detail with reference tothe accompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.References made to particular examples and implementations are forillustrative purposes, and are not intended to limit the scope of theinvention or the claims.

Generally, the term “about” as used herein unless specified otherwise ismeant to encompass a variance or range of ±10%, the experimental orinstrument error associated with obtaining the stated value, andpreferably the larger of these.

As used herein the term “high atomic number” refers to a chemicalelement with an atomic number (or proton number) of 72 or greater. Asused herein, chemical elements with an atomic number (or proton number)of less than 72 refers to as having a “low atomic number” or “loweratomic number.”

Various novel embodiments disclosed herein provide methods to improvethe adhesion of high atomic number metals and alloys to surfaces, suchas oxygen or hydroxyl rich surfaces, including but not limited to: metalsurfaces, films, fabrics, amongst others. Various embodiments providemethods to enable adhesion of high atomic number metals and alloys toglass surfaces, such as glass fiber fabrics. Various embodiments providemethods to enable adhesion of high atomic number metals and alloys topolymeric surfaces, such as polymeric fabrics, such as Nomex®, Kevlar®,cotton, nylons, and polyesters, which have hydroxy, carboxyl, amino,ketyl functional groups on the surface.

Various embodiments use a lower atomic number metal as a tie down, or asan adhesion promoter, thereby reducing oxide formation at the highatomic number metal interface. Various embodiments use lower atomicnumber metals coated onto oxygen and/or hydroxyl rich surfaces beforethe oxygen and/or hydroxyl rich surfaces are coated with high atomicnumber metals and/or metal alloys. Various embodiments use lower atomicnumber metals coated onto to glass surfaces, such as glass fiberfabrics, before the glass surfaces, such as glass fiber fabrics, arecoated with high atomic number metals and/or metal alloys. Variousembodiments use lower atomic number metals coated onto to polymericsurfaces, such as polymeric fabrics, such as Nomex®, Kevlar®, cotton,nylons, and polyesters, which have hydroxy, carboxyl, amino, ketylfunctional groups on the surface, before the polymeric surfaces arecoated with high atomic number metals and/or metal alloys. The coatingof the oxygen and/or hydroxyl rich surfaces with lower atomic numbermetals prior to coating with the high atomic number metals and/or metalalloys may form stronger mechanical interfaces on oxygen and/or hydroxylrich surfaces, such as metal surfaces, films, and fabrics. As such,various embodiments may enable the formation of layered materials havingstrong adhesion between a high atomic number metal and/or metal alloylayer and an oxygen and/or hydroxyl rich layer. As a specific example,various embodiments may enable formation of a strongly adhesively bondedtantalum coated glass fiber fabric. The embodiment strongly adhesivelybonded tantalum coated glass fiber fabric may include a metal tie downlayer, such as a lower atomic number metal tie down layer of titanium(Ti), palladium (Pd), etc., at the interface of the tantalum and glassfiber fabric. The use of the tie down metal layer with high atomicnumber metals in various embodiments enables methods to surface coatfabrics with high atomic number metals.

In various embodiments, a lower atomic number metal tie down layer (alsoreferred to as a strike layer) may be deposited onto a surface, such asan oxygen and/or hydroxyl rich surface, to promote adhesion of a higheratomic number metal or metal alloy layer to the surface, such as theoxygen and/or hydroxyl rich surface. In various embodiments, the metaltie down layer, may be a thin metal tie down layer that enables a higheratomic number metal to bond to the metal by forming an inter-metallic ordiffusion bond, having greater adhesion. In various embodiments, themetal tie down layer may be a layer of titanium (Ti), a layer oftitanium alloy, a layer of palladium (Pd), a layer of tin (Sn), a layerof bismuth (Bi)-tin (Sn) alloy (Bi—Sn), a layer of antimony (Sb)-tin(Sn) alloy (Sb—Sn), a layer of molybdenum (Mo), a layer of molybdenumalloy, a layer of aluminum (Al), a layer of aluminum alloy, a layer ofantimony (Sb), a layer of antimony alloy, a layer of palladium alloy, alayer of iron (Fe), a layer of iron-alloy (e.g., iron-molybdenum alloy,iron-copper alloy, stainless steel, etc.), and/or a layer of copper(Cu)-tin (Sn) alloy (Cu—Sn). Titanium and palladium have strong adhesiveproperties and form strong oxide bonds for use as a tie down or strikelayer. Both titanium and palladium can be used to alloy with high atomicnumber materials. Titanium alloys and palladium alloys also show strongadhesion with different metals. As one specific example, the titaniumalloy Ti-6Al-4V may be used to form the metal tie down layer. Palladiumand tin (Sn) can be used as an alloying tie down layer for plating.Titanium has a useful property with diffusing oxygen away from itssurface during diffusion bonding to itself or other metals. Thus,tantalum, and other high atomic number metals, form a strong interfacewith titanium by diffusion bonding. Additionally, alloys of bismuth andtin, alloys of antimony and tin, molybdenum, and molybdenum alloys mayprovide radiation shielding properties that may be beneficial for use ina tie down layer.

Various embodiments may use a metal tie down layer that may enable thepreparation of a strongly adhesively bonded tantalum coated glass fiberfabrics. One such embodiment may be used to prepare hybrid compositesthrough fiberglass processing with resins, such as vacuum assisted resintransfer molding, etc. In another embodiment, a strongly adhesivelybonded tantalum coated glass fiber fabric may also be coated with a thinflexible or elastomeric adhesive to form a flexible cloth material.Various embodiments may use a metal tie down layer that may enable thepreparation of a strongly adhesively bonded high atomic number metal ormetal alloy (e.g., tantalum, tantalum alloy, tungsten, tungsten alloy,lead, lead alloy, bismuth, bismuth alloy, etc.) coated glass fiberfabrics. Such an embodiment of a strongly adhesively bonded high atomicnumber metal or metal alloy (e.g., tantalum, tantalum alloy, tungsten,tungsten alloy, lead, lead alloy, bismuth, bismuth alloy, etc.) coatedglass fiber fabric may be used to prepare hybrid composites throughfiberglass processing with resins, such as vacuum assisted resintransfer molding, etc. Such an embodiment of a strongly adhesivelybonded high atomic number metal or metal alloy (e.g., tantalum, tantalumalloy, tungsten, tungsten alloy, lead, lead alloy, bismuth, bismuthalloy, etc.) coated glass fiber fabric may also be coated with a thinflexible or elastomeric adhesive to form a flexible cloth material.

Various embodiments may enable coating of glass fiber fabrics and otherpolymeric fabrics, such as Nomex®, Kevlar®, cotton, nylons, andpolyesters, which have hydroxyl, carboxyl, amino, ketyl functionalgroups on the surface, using a spray system, such as a thermal spraysystem. Examples of thermal spray systems suitable for use with thevarious embodiments may include RF plasma spray system, wire arcsystems, flame based systems, kinetic systems, etc. Various embodimentsenable delivery of coated high atomic metals and/or allows on fabricsusing a spray process, such as a thermal spray process (e.g., RF plasmaspray process, a flame process, a kinetic process, a wire arc process,etc.). Various embodiments may enable the formation of high atomicnumber metal layers on a flexible fabric.

FIG. 1 is a photograph showing a comparison of a sample radio frequency(RF) plasma sprayed tantalum (Ta) coated glass fiber fabric 102 withouta tie down layer to an embodiment sample RF plasma sprayed Ta coatedglass fiber fabric 103 with a RF plasma sprayed tie down layer ofTitanium (Ti) in accordance with the embodiments. In FIG. 1 , the topfabric 102 is a RF plasma sprayed tantalum coated glass fiber fabric.The bottom fabric 103 is a RF plasma sprayed tantalum coated glass fiberfabric with a RF plasma sprayed tie down layer according to the variousembodiments. The top fabric 102 has flaking metal at folded and cutareas. The bottom fabric 103 does not appear to have flaking areas atthe edges or in areas that had been folded or handled.

FIG. 2 is an illustrative process flow diagram 200 that may be partiallyor wholly implemented in the formation of a layered material. Theoperations of flow diagram 200 may be implemented to form a layeredmaterial having strong adhesion between a high atomic number metaland/or metal alloy layer and an oxygen and/or hydroxyl rich layer usinga lower atomic number metal tie down layer in accordance with certainembodiments. The layered material formed by operations provided withinprocess flow 200 may be used as an input material in preparingsubsequent articles, such as hybrid composites, flexible cloths, etc.

In block 201, a base material having a first surface may be provided.The first material may be a material (e.g., a fabric, a film, a sheet,etc.) with a first surface that is an oxygen or hydroxyl rich surface.The base material may be a glass material, such as a glass fabric. Thebase material may be a polymeric fabric, such as Nomex®, Kevlar®,cotton, nylon, or polyester, which has hydroxy, carboxyl, amino, ketylfunctional groups on the surface.

In block 202, a tie down layer may be deposited onto the first surfaceof the base material. The tie down layer may be metal or metal alloyparticles deposited onto the first surface. In various embodiments, thetie down layer may be deposited by spraying, such as by thermal spraying(e.g., RF plasma spraying, flame spraying, kinetic spraying, wire arcspraying, or other techniques). In some embodiments, the sprayingprocess, such as RF plasma spraying, etc., may melt the outer layer ofthe base material during the deposition process. In various embodiments,the tie down layer may be a thin layer deposited on the first surface.As an example, the thin layer may be about 1 millimeter (mil) thick. Asan example, the thin layer may be about a thickness equal to theparticle diameter of the material being deposited to form the tie downlayer. As such, the thin layer may be a single particle layer of the tiedown layer material, such as a lower atomic number metal. In variousembodiments, the tie down layer may be a lower atomic number metal ormetal alloy deposited on the first surface. As examples, the metal tiedown layer may be a layer of titanium (Ti), a layer of palladium (Pd), alayer of tin (Sn), a layer of bismuth (Bi)-tin (Sn) alloy (Bi—Sn), alayer of antimony (Sb)-tin (Sn) alloy (Sb—Sn), a layer of molybdenum(Mo), a layer of molybdenum alloy, a layer of titanium alloy, a layer ofaluminum (Al), a layer of aluminum alloy, a layer of antimony (Sb), alayer of antimony alloy, a layer of palladium alloy, a layer of iron(Fe), a layer of iron-alloy (e.g., iron-molybdenum alloy, iron-copperalloy, stainless steel, etc.), and/or a layer of copper (Cu)-tin (Sn)alloy (Cu—Sn). In some embodiments, the deposition of the tie down layerin block 202 may be a multiple part deposition process having an initialdeposition of particles followed by one or more subsequent depositionsof additional particles. In some embodiments, the tie down layer may beformed by depositing one type of metal or metal alloy particles in aninitial deposition and another type of metal or metal alloy particles ina subsequent deposition to form a tie down layer of two or moredifferent type metal or metal alloy particles. For example, particles oftitanium, palladium, tin, bismuth-tin alloy, antimony-tin alloy,molybdenum, molybdenum alloy, titanium alloy, aluminum, aluminum alloy,antimony, antimony alloy, palladium alloy, iron alloy, or copper-tinalloy may be deposited in an initial deposition part of step 202 anddifferent particles of titanium, palladium, tin, bismuth-tin alloy,antimony-tin alloy, molybdenum, molybdenum alloy, titanium alloy,aluminum, aluminum alloy, antimony, antimony alloy, palladium alloy,iron alloy, or copper-tin alloy may be deposited in one or moresubsequent deposition parts of step 202 to form a tie down layer of atleast two different type metal or metal alloy particles. As a specificexample, palladium particles may be deposited in an initial depositionas part of block 202 and then tin particles may be deposited in asubsequent deposition as part of block 202 and/or another process(es).Such a specific example of depositing palladium particles followed bydepositing tin particles to form the tie down layer may be useful whenthe high atomic number material to be deposited will be bismuth. Asanother specific example, when the high atomic number material to bedeposited will be bismuth, the tie down layer may be formed, such aspart of block 202, by depositing bismuth-tin alloy particles orantimony-tin alloy particles.

A high atomic number metal or metal alloy layer may be deposited ontothe first surface (such as for example, at block 203). The high atomicnumber metal or metal alloy layer may be deposited onto the firstsurface after depositing the tie down layer onto the first surface. Invarious embodiments, the subsequent high atomic number metal or metalalloy layer may be deposited by spraying, such as thermal spraying(e.g., RF plasma spraying, flame spraying, kinetic spraying, wire arcspraying, or other techniques.). In some embodiments, the spraying, suchthermal spraying (e.g., RF plasma spraying, or other techniques), maymelt the outer layer of the base material and/or the tie down layerduring the deposition process. As an example, the high atomic numbermetal or metal alloy layer may be a layer of tantalum (Ta) or tantalumalloy. As another example, the high atomic number metal or metal alloylayer may be a layer of tungsten (W) or tungsten alloy. As a stillfurther example, the high atomic number metal or metal alloy layer maybe a layer of lead (Pb) or lead alloy. As yet another example, the highatomic number metal or metal alloy layer may be a layer of bismuth (Bi)or bismuth alloy (e.g., bismuth alloy including tin, bismuth alloyincluding antimony, etc.). The high atomic number metal or metal alloylayer may form a strong adhesive bond by inter-metallic or diffusionbonding. The resulting material may be a multilayer material of thefirst material adhesively bonded with the high atomic number metal ormetal alloy using the tie down layer. The high atomic metal or metalalloy layer may be formed from particles of metal or metal alloy havinga high atomic number that is higher than the atomic number of theparticles of metal or metal alloy deposited to form the tie down layer.As such, the tie down layer may be formed of particles of lower atomicnumber metal or metal alloy and the high atomic number metal or metalalloy layer may be formed of particles of higher atomic number metal ormetal alloy.

As a specific example, one or more operations of flow diagram 200 mayresult in a RF plasma sprayed tantalum coated glass fiber fabric with aRF plasma sprayed tie down layer of titanium. A RF plasma sprayedtitanium layer may be deposited onto glass fiber fabric for use as a tiedown or strike layer for a subsequent RF plasma sprayed tantalum layer.The RF plasma sprayed titanium layer tie down layer may be about 1 milthick or equivalent to a thin layer of the titanium. The initialtitanium powder may be about the thickness of the average particlediameter, or slightly greater. This thin surface allows the tie downwith the minimal amount of titanium. The subsequent RF plasma sprayedtantalum forms a strong adhesive bond by inter-metallic or diffusionbonding. As such, the resulting layered material has a strong adhesivebond at the interface between the high atomic number metal or metalalloy layer and the first surface of the base material because theparticles of the metal or metal alloy of the tie down layer are presentat the interface supporting the inter-metallic or diffusion bonding.

FIG. 3 is a photograph of two samples of RF plasma sprayed tantalumcoated glass fiber fabric with a RF plasma sprayed tie down layer oftitanium in accordance with an embodiment. The two samples in FIG. 3were formed according to an operations incorporating teachings of flowdiagram 200. FIG. 4 is a scanning electron microscope (SEM) image of aTa/Ti glass fabric surface in accordance with an embodiment. FIG. 4shows the Ta/Ti glass fabric surface at a 200× magnification. The Ta/Tiglass fabric surface of FIG. 4 was formed according to teachings of flowdiagram 200. FIG. 5 is a SEM image of a cross-section of the Ta/Ti glassfabric shown in FIG. 4 at a 250× magnification. FIG. 6 is another SEMimage of a cross-section of the Ta/Ti glass fabric shown in FIG. 4 at a1000× magnification. FIG. 7 is a SEM image of a cross-section of anindividual glass fiber of the Ta/Ti glass fabric shown in FIG. 4 at a20000× magnification.

The preceding description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the following claims and theprinciples and novel features disclosed herein.

What is claimed is:
 1. A method for making a layered material,comprising: depositing a tie down layer comprising particles of at leastone of titanium or a titanium alloy onto a first surface of basematerial, the base material comprising a glass fabric; and depositing ahigh atomic number metal layer onto the first surface after depositingthe tie down layer comprising particles of at least one of tantalum or atantalum alloy.
 2. The method of claim 1, wherein at least one of thedepositing of the tie down layer and the depositing of the high atomicnumber metal layer comprises a thermal spraying process.
 3. The methodof claim 2, wherein the thermal spraying process comprises a radiofrequency (RF) plasma spraying process.
 4. The method of claim 1,wherein the first surface of the base material is an oxygen or hydroxylrich surface.
 5. The method of claim 4, wherein the particles of the tiedown layer further comprise a metal or an alloy selected from the groupconsisting of: palladium, tin, aluminum, aluminum alloy, antimony,antimony alloy, and combinations thereof.
 6. The method of claim 4,wherein a thickness of the tie down layer is about a thickness of anaverage particle diameter of the particles of the tie down layer.
 7. Themethod of claim 4, wherein a thickness of the tie down layer is aboutone millimeter.
 8. The method of claim 1, wherein: the particles of thetie down layer further comprise at least one of: palladium, tin,bismuth-tin alloy, antimony-tin alloy, bismuth-tin-antimony alloy,molybdenum, molybdenum alloy, iron, iron alloy, and copper-tin alloy;and the particles comprising the high atomic number metal layer furthercomprise at least one of: tungsten, tungsten alloy, lead, and leadalloy.
 9. A method for making a layered material, comprising: depositinga tie down layer comprising particles of at least one of titanium or atitanium alloy onto a first surface of base material, the base materialcomprising a glass fabric; and depositing a high atomic number metallayer onto the first surface after depositing the tie down layercomprising particles of at least one of tungsten, a tungsten alloy,bismuth, a bismuth alloy, lead, or a lead alloy.
 10. The method of claim9, wherein at least one of the depositing of the tie down layer and thedepositing of the high atomic number metal layer comprises a thermalspraying process.
 11. The method of claim 10, wherein the thermalspraying process comprises a radio frequency (RF) plasma sprayingprocess.
 12. The method of claim 9, wherein the first surface of thebase material is an oxygen or hydroxyl rich surface.
 13. The method ofclaim 12, wherein the particles of the tie down layer further comprise ametal or an alloy selected from the group consisting of: palladium, tin,aluminum, aluminum alloy, antimony, antimony alloy, and combinationsthereof.
 14. The method of claim 12, wherein a thickness of the tie downlayer is about a thickness of an average particle diameter of theparticles of the tie down layer.
 15. The method of claim 12, wherein athickness of the tie down layer is about one millimeter.